Swank apartments are begging for buyers on Manhattan's "Billionaires' Row" -- with more than 40% sitting unsold in towers that top out at 100 stories, The Post has learned.

Five years after the iconic One57 building became the city's first "supertall" residential skyscraper, only 84 of its 132 pricey condos have been bought -- leaving more than a third of them still on the market and none under contract, according to data compiled by leading appraiser and researcher Jonathan Miller.

Six other nearby buildings have as much as 80% of their units available, the figures show, with the total value of all the unsold inventory estimated by one analyst at between $5 billion and $7 billion.

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Lenz said the high prices were caused by a combination of factors, including the costs of property, construction, financing and high-end marketing -- and savvy developers who have clauses in their contracts that keep lenders from forcing them to drop prices, thereby cutting into their profits.